Identification systems for domestic animals which comprise a passive transponder unit attached to the animal are well-known, see e.g. the patent U.S. Pat. No. 4,510,495. In such a system, in the transponder unit receiver circuits are provided for receiving radio waves of high frequency and for storing energy drawn from these waves. Further there are transmitter circuits for transmitting, by means of such stored energy and by radio waves, identity information which uniquely identifies the transmitting unit and thereby the domestic animal to which the transponder unit is attached.
In the patent U.S. Pat. No. 4,247,758 for James A. Rodrian it is that a domestic animal such as a dairy cow is provided with a transponder unit which is connected to an electronic counter register powered by a battery for storing the number of signals obtained from a motion detector or sensor. When the transponder is located within a region which is reached by radio signals from a questioning unit, the transponder unit transmits data by means of radio signals data, this data comprising information which constitutes an identification number of the domestic animal and also indicates the number of movements that the domestic animal has made. By evaluating this latter transmitted number it is possible to decide whether the cow is or is not in the estrus period.
In the European patent having the publication No. EP-B1 0 087 015, also for James A. Rodrian, a similar device is disclosed where the transponder unit instead transmits data indicating whether the number of counted movements during a predetermined time period exceeds the reference value by a predetermined amount.
A transponder unit having a motion sensor and a counter for the number of movements is also disclosed in the patent U.S. Pat. No. 4,618,861 for John W. Gettens et al., where the supply of electrical current instead is provided by the way that the motion sensor in the conventional way generates electrical pulses and by storing and utilizing the energy of the pulses for the transmission of information.
In the use of passive transponders having no interior own current supply, at each operation thereof energy must be provided from the outside. It is made, as has been mentioned above, usually by means of high frequency energy transferred by radio waves, the energy being stored in a capacitor in the unit. The stored energy is then used for transmitting a radio signal carrying some type of desired information. For natural reasons such high frequency energy can only be transferred over relatively restricted distances, of the magnitude of order of one or a few meters at most. When an identification system also is used for collecting data, for example for detection of the number of movements or accelerations, some type of an active or more powerful current supply must be arranged, at least for driving a clock and a register, in which the collected data are stored. Therefore, when such collecting of data is to performed, in most cases a battery is required. A battery has a limited useful lifetime, which is a distinctive disadvantage for identification units, which in the ideal case are to be attached to the animal and are to remain there and be operative for a long time period. In certain cases the motion sensor, see the mentioned patent for John W. Gettens, can itself, having no current supply from the outside, generate the pulses which are used for modifying a register or a memory.
For identification devices which are to transmit their information over long distances, it must be possible to transmit sufficiently strong radio signals and therefor a current supply of the type battery is required. It can be particularly important in the detection of estrus in cows, since they can go loosely during the non-milking period and are seldom collected to some feeding place or similar place. Such a transmission of information over long distances utilizing the energy of a battery should however be executed as seldom as possible, since in at each transmission occasion relatively much energy is consumed.